It had already been a bad week for West Ham, featuring yet another deflating conclusion to a transfer window and the dismissal of their head of player recruitment following his remarks about African footballers, and the atmosphere surrounding them had descended to new levels of toxicity once it was over.

For David Sullivan and David Gold, West Ham’s deeply unpopular owners, the temptation to close their eyes and stick their fingers in their ears must have been overwhelming long before the final whistle of this damaging defeat. “RIP WHU” was the message on one banner in the away end, where the travelling supporters needed little encouragement to increase the volume of their protests.

West Ham offered meek opposition in the face of a robust performance from Brighton, whose first victory since 23 December eased their relegation fears, and it was a match when the visitors’ self-destructive tendencies in the transfer market were ruthlessly exposed.

David Moyes spent a dismal second half turning to backroom staff in despair. West Ham had travelled to the Amex Stadium hoping for some respite after sacking Tony Henry, their transfer chief, on Friday night.

With their messy efforts at strengthening their squad still fresh in the memory, the mood darkened when Brighton scored after eight minutes. Chris Hughton’s team made a pulsating start and it was a slick counterattack that culminated in Glenn Murray beating a shoddy offside trap.

The move developed at impressive speed after a West Ham attack fizzled out and Brighton were grateful to Roger East, the referee, for playing on when Aaron Cresswell brought down Anthony Knockaert with a poor tackle.

Unfortunately for Cresswell, the ball was already nestling in West Ham’s net by the time he was booked. Pascal Gross released Murray, who was played onside by Declan Rice, and the striker slipped the ball underneath Adrián to claim his fifth goal in his past seven matches. Both managers denied any knowledge of Cresswell and Knockaert clashing in the tunnel at half-time.

West Ham were protecting a six-match unbeaten run in the Premier League and they crept back into the game, equalising through Javier Hernández in the 30th minute.

Brighton regained control after the break and were rewarded for their pressure when José Izquierdo stunned the visitors in the 59th minute. West Ham failed to clear a corner and the winger took a touch before whipping a magnificent effort into the far corner from the left of the area. “I knew we had to raise our game again,” Hughton said. “I thought the second half was close to being as good as we’ve been this season.”

Izquierdo celebrated wildly and was booked for whipping off his shirt. Brighton did not mind. Despite introducing Michail Antonio, who was dropped for last Tuesday’s draw with Crystal Palace for arriving late to a team meeting, West Ham lacked guile without Marko Arnautovic and Manuel Lanzini. They are three points above the bottom three – Brighton trail them on goals scored – and are in danger of losing momentum.

“We just never seemed to get a grasp of the game in the second half,” Moyes said. “It was disappointing.”

Moyes, who denied that Henry’s dismissal had distracted his players, was preparing to give Jordan Hugill his debut with 15 minutes left. But the striker’s jog up the touchline merely granted him a better view of Gross thumping a superb shot past Adrián from 18 yards.

Brighton had failed to score in nine of their previous 13 league matches, yet they have scored six in two games against West Ham.

“Where’s the money gone?” the West Ham fans chanted. It was a good question.